In U.S. Pat. No. 4,634,850, Pierce et al. describe an optical data storage medium having a data layout in the form of contiguous nonoverlapping rows and columns of memory cells. Data is recorded as optically contrasting spots, representing bits, in the memory cells. The presence or absence of a spot in any particular memory cell thus represents either a zero or one. Rows of cells are read by a linear CCD array, one row at a time as the media advances. A servo system uses lines of track marks at the ends of the rows for proper alignment of the media with the CCD array.
Another data storage system of the prior art is shown in FIG. 1, a read-only memory card 11 has data digitally encoded into a pattern of reflective (or transmissive) and nonreflective (or absorptive) areas 13 and 15 aligned in tracks 17. A card reader 21 makes use of a single light beam 23 focused to a spot 25 on the card 11 which in size is equal to or smaller than the size of the pattern areas 13 and 15 contained on the card 11. The light source 27 can be a laser, a light emitting diode or an incandescent bulb. Collimating and focusing optics 29 and 31, as well as a beamsplitter 33, may be used to direct and focus the beam 23 to spot 25. The card 11 or the beam 23 or both are moved relative to one another so the spot 25 scans the length of a track 17. Light reflected from the card 11 is detected by a light detector 35, which generates an electrical signal 37. Detector 35 can be a photocell, photomultiplier or any other device capable of converting light to an electrical signal. In this card reader 21, the data pattern areas 13 and 15 are passed under the focused light beam 23 without the aid of any servo mechanism to track the pattern as it passes by. However, the initial x-direction or cross-track placement of the beam spot 25 relative to a data pattern track 17 may be controlled with the aid of a marker 19 at the beginning of each track 17 so that the spot 25 is centered on the track or before the reading operation. This type of reader has an advantage of being simpler and less costly than those having servo track following mechanisms.
In such a prior art system with no servo mechanism and centering only at the start of a data pattern track, the pattern areas 13 and 15 must be made sufficiently large so that mechanical tolerances of both the read-only memory card 11 and the card reader 21 do not affect the integrity of the retrieved data. In particular, because there is no servo mechanism to bring a deviating beam spot back to the middle of a track, one must take skew into account. Skew can be due to both card manufacturing tolerances and reader mechanical tolerances. The conventional approach uses data pattern areas which are rectangular bars aligned in long rectangular tracks. The width of the bars, and therefore of the tracks as well, is calculated based upon the worst case skew angle .theta..sub.max, seen in FIG. 1A. The data pattern width is made equal to twice the maximum possible deviation .delta..sub.max from center at the end of a scan, where .delta..sub.max =Ltan.theta..sub.max and L is the length of a track. Unfortunately, while relatively skew insensitive, this approach limits the data storage capacity for this type of system.
An object of the present invention is to provide a data storage medium for use with read systems with no servo track following which has a greater data storage density but is still relatively insensitive to skew during reading.
Another object of the present invention is to provide a method of encoding a data pattern on a data storage medium in a skew insensitive manner and with increased storage density.